P.L. de Vaal

Advanced process monitoring using an on-line non-linear multiscale principal component analysis methodology

Abstract A nonlinear multiscale principal component analysis (NLMSPCA) methodology is proposed for process monitoring and fault detection based on multilevel wavelet decomposition and nonlinear principal component analysis via an input-training neural network. Performance monitoring charts with non-parametric control limits are applied to identify the occurrence of non-conforming operation prior to interrogating differential contribution plots to help identify the potential source of the fault. A novel summary display is used to present the information contained in bivariate graphs in order to facilitate global visualization. Positive results were achieved through assessing the capabilities of the monitoring scheme on a nonlinear industrial process.

Using MV overshoot as a tuning metric in choosing DMC move suppression values

Traditionally the tuning of dynamic matrix control (DMC) type multivariable controllers is done by trial and error. The APC engineer chooses arbitrary starting values and tests the performance on a simulated controller. The engineer then either increases the values to suppress movement more, or decreases them to have the manipulated variables move faster. When the controller performs acceptably in simulation, then the tuning is improved during the commissioning of the controller on the plant. This is a time consuming and unscientific exercise and therefore often does not get the required attention. This leads to unacceptable controller behaviour during commissioning and sub-optimal control once commissioning is completed. This paper presents a new method to obtain move suppression factors for DMC type multivariable controllers by using a Nelder Mead search algorithm to find move suppressions that will provide acceptable control behaviour. Acceptable behaviour is described by characterising the dynamic move plan calculated by the controller for each of the manipulated variables.

A Constant Temperature FZG Test for Open Gear Lubricants

The 12-stage FZG test (A/8,33/90) was originally developed to determine scuffing load capacity of engine oils [1]. During each load stage, the test is started at the same initial sump temperature. Heat of friction generated during a load stage results in a temperature increase of the lubricant. Final sump temperatures as high as 140 °C are often reached after the final load stage (Load Stage 12). A modified version of the test, (A/2,76/50) is used for laboratory determination of open gear grease performance [2]. In open gear lubrication, typical gear operating temperatures are in the range 50°C–80°C. Test temperatures higher than this may give rise to different wear characteristics than would be found under typical operating conditions. It would therefore be of practical interest to have a laboratory-based means to determine the temperature-dependent characteristics of an open gear lubricant. A compact heat exchanger was designed, built and inserted in the FZG test bed. By manipulating the flowrate of an external cooling medium, the temperature of the test fluid in the sump can be maintained at a desired temperature. Results for four representative open gear lubricants were obtained using the constant temperature test and compared with results obtained with the conventional test. These results indicated that the new test can discern between samples that perform similarly during the conventional test.© 2005 ASME

Biorefining: A Green Tribological Perspective

As a developing country, South Africa is in a unique position with regard to establishing, maintaining and expanding infrastructure to ensure compliance with international trends with respect to environmental regulations, while at the same time establishing the means to provide access to affordable energy to all its citizens to share the potential of its resources. In many respects, tribology plays an important role in saving of energy as well as ensuring that requirements with regard to protecting the environment are complied with. Green tribology can rightly be regarded as an approach which is timely and which has an impact on many activities like electricity generation, production of synthetic fuels and lubricants, mining operations and protection of the environment and its resources. Focusing on the interface between Tribology and Biorefining, several interesting possibilities open up. With the constant rise in the price of oil, alternatives to crude oil as primary energy source and as basic feedstock for fuels and chemicals are becoming more and more attainable. In this chapter an overview is provided of the above, from a South African perspective. A number of case-study examples are given which indicate that a “green” approach in finding engineering solutions to tribological problems which could have a far-reaching impact on the environment. Three examples are used, namely how proper selection of tailor-made lubricants could decrease energy usage in gear-driven systems. The focus here is on the power industry, where coal-based power plants are the only economically feasible solution to the increasing demand for electricity in a developing economy with virtually no crude oil reserves. The success atttained in this endeavour should stimulate similar projects in the mining sector of the country. In the second instance, ingenious application of tribology with respect to application of specialised lubricants from a renewable source, namely plant oils, can decrease cost of lubrication and, in addition, can resolve difficult issues with regard to disposal of contaminated waste in metal cutting operations, indicating the value of a “green tribology approach”. Thirdly, combining the concept of biorefineries, tribology and the ability to synthesise products to suit specific requirements, including formulation of lubricants and fuels, can lead to substantially improved products, impacting in a positive way on the environment.

Mechanisms of Lubricated and Unlubricated Friction and Wear

A study was undertaken to analyse results obtained with the HFRR instrument by identifying crucial factors contributing to friction and wear mechanisms. For lubricated tests a mixture of n-hexadecane (cetane) as base fluid and palmitic acid as lubricity additive was used. Experimental results, including results obtained by running unlubricated (dry) tests and results obtained from literature were all considered to formulate the hypothesis that a fully functional additive film will not survive the full duration of the lubricity test, which is 75 minutes [5]. Friction values obtained in the experiments conducted were much higher than what is expected for boundary films formed by long chain carboxylic acids. Comparable friction values were obtained in the first few seconds of the tests, but the friction values increased indicating the destruction of the boundary film. Metal oxide lubrication dominates for the remainder of the test.Copyright

The Role of Synovial Fluid Properties in a Dynamic Hip Joint Simulation

Accurate mathematical modelling of joints is directly related to the need to obtain a better fundamental understanding of the role that elements of the system have on the dynamic tribological behaviour of the component it represents. Simulation models also make a contribution towards reduction of the cost of in-situ experimentation in biotribological systems, as well as overcoming difficulties with ethical and practical problems related to in-situ testing. The aim of the current work is to report on the results obtained from a dynamic model of a human hip joint. The model is used to investigate the tribological performance of the synovial fluid over a varying range of operating conditions — specifically pressure, temperature and varying load conditions. The model is based on a minimum of empirical relationships, preferring relationships grounded in solid fundamental theory. This reduces the number of parameters that can be modified in the model and ensures that predictions made are not based on external manipulations.Copyright

Implementation of a model based controller on a batch pulp digester for improved control

Abstract Effective control of the sulphite pulp digestion process in the production of dissolved pulp in a batch digester is limited by three important restrictions namely, the inability to measure the degree of polymerisation (DP) of the cellulose in the wood pulp, which is the controlled variable, the fact that flowrate of steam to an external heat-exchanger, through which the cooking liquor circulates, is the only manipulated variable available and that, due to scheduling requirements, cook time per digester is fixed. The traditional S-factor prediction of cook-time to control DP is inadequate. Use of a simplified model-based inferential technique to estimate DP offers an improved methodology to control the process. A simplified fundamental model with adjustable parameters was developed and its accuracy to predict DP based on given operating conditions was tested using available plant data. This model was built into a control structure for implementation on an operational batch digester. Use of this model enables adaptive response to changing conditions on the plant by optimal adjustment of the model parameters to fit the measured characteristics of the digester. Because the parameters form part of fundamental relationships in the model, realistic bounds can be placed on them during the optimisation process, enabling a better understanding of the behaviour of the model. By automating the optimisation process, the controller becomes independent, requiring no human intervention for day-to-day operation. The plant model is periodically adapted by adjusting the parameters in the model, based on historical data representing the performance of preceding cooks.

Controlling a nonlinear process with variable dead-time: A computer-based laboratory experiment

Abstract In process control, nonlinearities and dead-time often play a significant role in the unsuccessful implementation of control systems. This is due to variations in operating conditions and process behaviour affecting controller performance within the stability limitations of the feedback loop. In a laboratory experiment, where a liquid is heated by means of submerged electrical heating elements, temperature control of liquid in a tank by means of a feedback control loop can be investigated. The relationship between the manipulated variable and the tank temperature is non-linear. The loop was designed to contain significant dead-time, dependent on the variable flowrate of liquid through the system. By means of experimentally obtained open loop process reaction curves and by continously measuring the volumetric flowrate, enough information about the process can be obtained to develop a process model compensating for the presence of dead-time and to take the nonlinear behaviour of the process into account. Using a digital measurement and control system, an algorithm was developed which can be used for accurate control of the process. In this paper emphasis is placed on the current ability of computing equipment to apply theoretical concepts which had been known for a long time.